1. Technical Field
This Patent Document relates to SERDES (serializer/deserializer) communications in which the receiver operates with data re-timing/reclocking.
2. Related Art
SerDes communications links experience high frequency distortion (phase and amplitude) between the XMTR (transmitter) and RCVR (receiver) over a lossy channel. This distortion is manifested at the RCVR as intersymbol interference (ISI), i.e., a smearing of the transmitted data bits/symbols.
Channel equalization (EQ) can be used to counteract ISI distortion. Transmit (TX) pre-emphasis EQ pre-distorts the transmit signal, pre-emphasizing the high frequency leading edge bit transitions to reduce ISI effects at the RCVR. TX pre-emphasis EQ can be combined with receiver equalization such as CTLE (continuous time linear equalizer).
FIG. 1 illustrates a SerDes com link 100 with a XMTR 101 and a RCVR 107, configured for SerDes communication over a com channel 105. XMTR 101 generates serialized data, including providing TX pre-emphasis. Signal pre-emphasis can be implemented with a FIR (finite impulse response) channel driver in which pre-emphasis EQ is determined by FIR coefficients.
RCVR 107 is illustrated as a retiming receiver that includes in the RX Path a re-timing element 110 that re-times/re-clocks received data for input to an RX ASIC 120. Re-timing elements (such as retimers or reclockers) modify data edge timing based on a recovered or locally generated bit clock, and can be implemented with FIR signal output based on loadable FIR coefficients.
TX pre-emphasis EQ using FIR techniques is based on ISI cursor functions translated into FIR coefficients. Referring to FIGS. 2A/2B, ISI can be characterized in terms of an impulse response measured at the RX end. The ideal impulse for a current bit defines a main cursor, with the impulse response including energy in the adjacent bit intervals: pre-cursor energy of a previous bit that affects the current bit, and post-cursor energy of the next bit that affects the current bit.
SerDes com systems can use link training to adjust TX pre-emphasis EQ for channel conditions, improving ISI performance. For link training, the RCVR signals back to the XMTR link training information used by the XMTR to adjust TX pre-emphasis EQ, for example incrementing or decrementing TX pre-emphasis EQ, such as by adjusting FIR coefficients. In FIG. 1, this link training operation is illustrated in the TX signal waveforms as a change in post cursor increment.
However, conventional link training will not operate with SerDes corn links configured with a retiming receiver. Re-timing elements (RTEs) regenerate the received data without ISI equalization effects, i.e., without propagating to the output of the re-timing element any signal characteristics of the TX pre-emphasis EQ embodied in the data received at the input to the re-timing element (RTE). Thus, in FIG. 1, data re-timing/re-clocking prevents the RX ASIC from extracting any ISI cursor information representative of any change in TX pre-emphasis EQ (post cursor increment). In the context for FIR signal generation in the XMTR and the RCVR, re-timing/re-clocking prevents FIR cursor information (as reflected in FIR coefficients) for the received TX signal from propagating to the RX ASIC, through re-timing element.